A nondestructive measuring system

ABSTRACT

A system for nondestructively measuring the thickness of a surface stress layer of a glass material by measuring an increase of the fundamental frequency of a quartz resonator due to the stressed surface layer.

Ullllbu "gr g [15] f r Kammer 1 Feb. 22, 1972 [S4] A NONDESTRUCTIVE MEASURING 2.903.886 9/1959 Renaut ..73/67.2 SYSTEM 3,499,318 3/1970 Bogdanov et ul. 73/67.? r 2,439,130 4/1948 Firestone TA/07.8 [72] Inventor: Erwin W. Kalnmer, McLean, Va. [73] Assignee: The United st f Ameflca as Primary Examiner-Richard C. Queisser represented by the Secretary f the Navy Assistant Examiner-Arthur E. Korkosz Attorney-R. S. Sciascia, Arthur L. Branning, M. L. Crane and [22] Filed: Sept. 15, 1969 R Erickson [21] Appl.No.: 858,020 57 ABSTRACT [52] U.S.Cl. ..73/67.2,73/67.8R 1 [51] Int. Cl. ..G0ln 29/00 A system for nondestructively measuring the thickness of a [58] Field of Search ..73/67.2 surface stres layer of a glass material by measuring an increase of the fundamental frequency of a quartz resonator due [5 6] References Cited to the stresseTsTfr'facelayer. Ms. W UNITED STATES PATENTS 2 Claims, 1 Drawing Figure 2,605,633 8/1952 Gow ..73/67.2

l l 'irf nf "P' J/ ,5 K: 1 l\.,vIP" ff I3 OSCILLATOR m RECEIVER A NONDESTRUCTIVE MEASURING SYSTEM STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore.

BACKGROUND OF THE INVENTION This invention is directed to a system and method of nondestructively testing a glass material and moreparticularly to testing a glass material to determine the thickness of a surface layer in a state of compressive stress.

Heretofore, various systems have been used for nondestructive testing. Such systems determine thickness of a metal plate which may or may not have a backing, other systems determine thickness or flaws within an element tested. These prior art systems use an input which is varied over a specific range of frequencies to determine the reso nant freqgency ofthe test sample. From a determination sf t he resonant frequency the thickness or flaw may be determined.

SUMMARY OF THE INVENTION Many glass materials may be made more resistan failure initiated by the presence of surface cracks o fflaws f the surface layers are in a state of compressive stress.

It has been deiermined that the p ceoftransformed surface layers of polycrystalline glass suchas Pyroceram" inc? s e thev elocity of an ultrasonic wave as the compressional ar'shsafivavisiraiisntitidifirugh the material. The elastic properties associated with an ip gr ease in velocityproduces a fundamental frequency change H a bonded piezoelectric resonator which can be measured and then comparedwith a known scale to determine thewthickness of the transformed surface layer. The system herein shown and described is suitable for applying an ultrasonic wave to a test material and then comparing the detected frequency of the wave received from the crystal with that of a known to determine thickness of the surface layer.

STATEMENT OF THE OBJECTS It is therefore an object of the present invention to nondestructively determine the thickness of the outer surface layer of a material when this layer has distinctly different elastic parameters from the substrate.

Another object is to measure a glass material to determine the state of compressive stress of its outer surface and the thickness ofthe surface layer under stress.

Other objects and advantages of this invention will be obvious to those skilled in the art upon consideration of the following drawing and specification.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a block schematic diagram of an embodiment ofthe invention.

DETAILED DESCRIPTION Now referring to the drawing, there is shown a block schematic diagram of a system for carrying out the invention. As shown, a power supply such as a battery for portable equipment or any other suitable power source is connected to an oscillator-driver 11, preferably transistorized. The oscillator 11 is connected to a piezoelectric transducer 12 by input electrodes for impressing an adjustable frequency onto the transducer. The transducer is provided with output electrodes for determining an electrical output therefrom which is directed to a frequency measuring device such as a stable communication type radio receiver or resonance detector 13 which includes the necessary amplification circuitry. Thqggeillator frequency may be controlled by the piezoelectric transducer 'uency supplied to the test sample is a characteristic: frdqiihcy determined bythe substrate or the modified surface coupled to the transducer. For situations in which the oscillator-resonator system is not operated as a self excited oscillator, then the oscillator frequency is adjusted to fundamental resonance frequency of the bonded transducer, this resonance condition being detected by a suitable modification in the receiver and the resonance frequency measured as before.

In operation, the transducer is bond ed to the test sample 14 in which the portion' abave ih dotted line represents the layer having changed elastic properties. A fundamental frequency in the range of 10 mc./s. or 5 mc./s. is applied to the transducer. The electrical oscillations are converted into mechanical vibrations by the piezoelectric crystal and the mechanical vibrations are transmitted into the test material. M9r5-; rjg id sstisszezssrtia o m a fqrmsd. Surface l e 9 th test material will be evident ra min I I i mechanical vibrations at which the trans nantl'he output from the t r aiisdiic ei is irected t o a comiiier- Eial type radio receiver or frequency counter which is then tuned to the output frequency of the transducer to provide a measure of the output. Any other frequency monitor or resonance detector may be used to determine the frequency output; however, the commercial type radio has been specified to provide an inexpensive type monitor.

In carrying out the teaching of the invention, the transducer system is bonded 0 the surface of a gla erial which has WW..- "QIJEQDJIEQIEB. assassinate same transducer is th onded to like glass materials which have been treated to known but different thicknesses of transformed surface layers to obtain a list of frequency outputs for the different known transformed layers. Thus, a test made on an unknown may be compared with the list previously obtained to determine the thickness of the transformed layer of an unknown thickness. It is essential that the bonded transducer be chosen such that the corresponding sonic wavelength in the test material be substantially greater than [about 50 times] the anticipated range of thickness to be measured.

It has been determined that X-cut or Y-cut quartz resonators having a diameter of one-half inch and with a fundamental frequency of 10 mc./s. or 5 mc./s. respectively are suitable for bonding to a lithium treated surface of Pyroceram" to test for surface layer thickness. Such resonators displayed a sensitivity factorof +0.47 percent change in frequency for each mil layer thickness. Thus, a layer 6.5 mils thick correlates with a positive frequency change of over 300 kc./s. for a 10 mc. X-cut resonator, which is readily measurable with a stable communication type radio receiver.

A short discussion of transformed polycrystalline glass is found in an article, "Vibroanalysis of Materials" (Detection and Measurement of Residual Stress), by E. W. Kammer et al., Report of NRL Progress, Jan. and Mar. 1969, pages 24-26, and 19-20, respectively.

The system and method has been presented for testing a material to determine the thickness of a stressed surface layer. It is conceivable that applications may arise where the surface layer has become weaker elastically and would therefore have the affect of IoWeringtheresonant frequency of the material. 'r'iiissystfii may be used for determining weaker surface layers by measuring a frequency which is less than that for a normal material. Therefore the system may be used for rejecting materials which processed to make the surface layer weaker than that of the normal untreated material.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

l. A method of nondestructively testing a material to determine the thickness of a stressed surface layer in the material which comprises,

bonding one face of a transducer to a material to be tested,

applying a constant oscillating voltage of an adjustable frequency to said transducer to produce a resonant funwherein each mil layer thickness of a stressed surface layer results in an increased change in frequency of 0.47 per cent. 2. A method as claimed in claim 1 wherein,

said transducer is operative to produce a sonic wave length in the test material which is at least 50 tirr es the a nticipated layer thickness to be measured. 

1. A method of nondestructively testing a material to determine the thickness of a stressed surface layer in the material which comprises, bonding one face of a transducer to a material to be tested, applying a constant oscillating voltage of an adjustable frequency to said transducer to produce a resonant fundamental frequency vibration in said transducer whereby said transducer produces a corresponding sonic wave in said test material, detecting and measuring an output frequency produced by said transducer, and comparing the measured output frequency with a known output frequency to determine thickness of a stressed surface layer on the material to which the transducer is bonded, wherein each mil layer thickness of a stressed surface layer results in an increased change in frequency of 0.47 percent.
 2. A method as claimed in claim 1 wherein, said transducer is operative to produce a sonic wave length in the test material which is at least 50 times the anticipated layer thickness to be measured. 